Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing, or support layer; one or more unexposed photocurable layers; a protective layer or slip film; and often a protective cover sheet.
The support sheet or backing layer lends support to the plate. The support sheet, or backing layer, can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. Generally the most widely used support layer is a flexible film of polyethylene terephthalate that is typically about 5 mils or so thick. The support sheet can optionally comprise an adhesive layer for more secure attachment to the photocurable layer(s). Optionally, an antihalation layer may also be provided between the support layer and the one or more photocurable layers. The antihalation layer is used to minimize halation caused by the scattering of LTV light in the non-image areas of the photocurable resin layer.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Preferred photocurable materials include an elastomeric compound, an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahari, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,630; 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gmetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. More than one photocurable layer may be used. When only a single photocurable layer is present, it may be anywhere from about 25-275 mils thick.
In some plates, there is a second photocurable layer (referred to as an “overcoat” or “printing” layer) atop this first, base layer of photocurable material. This second layer usually has a similar composition to the first layer, but is generally much thinner, being on the order of less than 10 mils thick.
The photocurable materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of affecting a chemical change in an exposed moiety thereby causing crosslinking, polymerization, or curing. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the 1N and violet wavelength regions. One commonly used source of actinic radiation is a mercury arc lamp, although other sources are generally known to those skilled in the art.
The slip film is a thin layer (typically about 0.1 to 1.0 mil in thickness), which protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light. In normal use, the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photocurable material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter. The resulting surface has a relief pattern that reproduces the image to be printed. The relief pattern typically comprises a plurality of dots, and the shape of the dots and the depth of the relief; among other factors, affect the quality of the printed image. After the relief image is developed, the relief image printing element may be mounted on a press and printing commenced.
Exposure of the printing plate is usually carried out by application of a vacuum to ensure good contact between the negative and the plate. Any air gap will cause deterioration of the image. Similarly, any foreign material, such as dirt and dust between the negative and the plate results in loss of image quality.
Hot spots are a phenomenon that occurs with analog plates. Upon exposure, an air bubble can become entrapped between the negative and the slip film (on top of the plate). This air bubble causes a distortion of the light going to the plate and, in addition, can cause the negative to rise up slightly resulting in off-contact. The result is what is known as a “hot spot” and it manifests itself in the form of a reverse closing in or a feature disappearing. The hot spot essentially results in a loss of resolution in a specific region.
Attempts to get around hot spots have focused on reducing the exposure time or even by reducing the exposure time in a specific region of the plate.
The hot spots occur for a variety of reasons, including the following:                1) Imaging latitude;        2) Thickness of the negative;        3) Temperature of exposure, or ability of exposing unit to dissipate heat; and        4) Type of slip film (as discovered by the inventors here).        
Firstly, the imaging latitude of a given plate makes the plate more or less vulnerable to hot spot formation. The more energy that a given plate can accept and still hold a reverse, the less likely a hot spot will occur with that plate, or the less likely an occurring hot spot will affect that plate.
Second, the thickness of the negative can have a detrimental effect on the occurrence of hot spots. In general, the thicker the negative, the less likely the occurrence of hot spots. In addition, certain markets, including South America, prefer a low-cost, thin negative. Thus, hot spots can be more common in these markets.
Another common aspect of the low cost market is the use of exposure units that do not have an adequate cooling system in place, which also results in hot spots.
Finally, the type of slip film employed can also result in hot spots, as discovered by the inventors here. The inventors have surprisingly found that polyamide, which is commonly used as a slip film in flexographic printing elements produces hot spots. Based thereon, the inventors of the present invention have identified several other types of polymer systems and additives that may be used to formulate slip films and that are capable of substantially preventing the formation of hot spots.